Structure and Function of Lipoproteins
Lipoproteins are classified based in three ways. One of them is density. They are separated by Ultracentrifugation. There are five major groups of lipoproteins that have been identified that are important physiologically and in clinical diagnosis. Low-density lipoproteins density lies between 1.019-1.063 and average diameter ranges from 18-28 nm. High-density lipoproteins density ranges between 1.063-1.121 and the average diameter varies between 5-15 nm. Lipoproteins with high lipid content will have low density, larger size and float on centrifugation. Lipoproteins with high protein content sediment easily, have compact size and have a higher density. They are also classified based on electrophoretic mobility, which is mainly dependent upon protein content. The more protein content a lipoprotein has the faster it will move, and those with minimum content will have minimum mobility. Approximately 50% of the weight of LDL is cholesterol and only 25% is protein. HDL particles consist of 20% cholesterol and 50% protein. Since protein is more dense than fat, HDL particles are more dense than LDL particles, which explains the names "high-density" and "low-density" lipoproteins. The other major structural difference between LDL and HDL relates to the types of protein they contain. LDL contain proteins called B-100 proteins, while HDL particles contain mostly A-I and A-II proteins.The function of LDL is to deliver cholesterol to cells, where it is used in membranes, or for the synthesis of steroid hormones. Cells take up cholesterol by receptor-mediated endocytosis. One of the important functions of HDL is to transport cholesterol from the cells and tissue back to the liver. High HDL-cholesterol is good as it takes cholesterol out of cells and the blood and helps to prevent excess cholesterol. HDL also removes cholesterol deposited in the walls of blood vessels.